Process for preparing silica or silica-based thick vitreous films according to the sol-gel technique and thick films thereby obtained

ABSTRACT

A sol-gel process allowing the preparation, on a substrate, of glassy films of silicon oxide or mixed oxides based on silicon oxide, of thickness above 1 micrometer, generally between 2 and 20 micrometers and characterized by absence of defects, that turn out to be particularly suitable as waveguides in flat optical devices.

This application is the national phase of international applicationPCT/IT99/00284 filed Sep. 6, 1999 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to a sol-gel process for the preparationof thick glassy films of silicon oxide or based on silicon oxide and tothe thick films thus obtained.

In the technology of solid state, with the term “film” is meant a thinlayer of a material having a thickness generally comprised between a fewtens of nanometers (nm) and a few tens of micrometers (μm), said layerbeing supported over a substrate of another material, generally a flatgeometry.

The term “thick” typically refers to films of thickness larger than 1μm.

Thick glossy films, deposited over a suitable substrate, are the objectof extensive research in view of their foreseen use in the field oftelecommunications, particularly telecommunications on optical andelectro-optical cables.

In the past, telephone communications and data transmissions wererealised transforming the signal into electronic impulses that weretransmitted by means of cables of an electrically conductive material,generally copper.

Nowadays, in particular for the long distances, transmissions onelectrical cables have been almost completely replaced by transmissionsan optical fibers. As known, the optical fibers are glassy fibers whosestructure comprises at least a central part, called nucleus, and anouter part, called mantle, made of glasses having slightly differentchemical compositions; the different chemical composition gives rise toa difference in the refractive index of the two materials that allowsconfining the optical signal in the nucleus. Commonly the mantle is madeof pure silicon oxide, whilst the nucleus is made of a mixed oxide basedon silicon oxide containing from a few percent to about 10% by mole ofdifferent oxides such as germanium oxide.

The optical fibers offer several advantages over electrical cables asmeans for information transmission, such as a lower level of noise andlower signal attenuation as well as a higher amount of informationtransmitted per unit time, resulting in a higher transmission rate.

Despite these advantages, it has not been possible so far to fullyexploit the potentiality of optical communications: in fact, a completecommunication system requires devices for processing signals, forinstance for transforming voice into signal at the two ends of the cablein telephonic transmissions, or for amplifying the signal along thefibre, that is rendered necessary due to unavoidable attenuation of thesame signal. More generally, the so-called operation of signalcommutation that is needed for delivering the same signal in the networkrequires suitable devices.

To this end, traditional electrical devices (electronic switches) arepresently used, and generally any operation on the signal requires aconversion into electrical signal followed by a possible furtherconversion back to optical signal. In these operations time and signalquality are lost. As a consequences, a strong need is felt for opticalor electro-optical devices capable of guiding an optical signal as wellas of performing on it commuting operations comparable to those operatedby electronic devices on electrical signals.

The main features that optical devices must have, are:

material of very high transmittance, requiring absence of inclusions andmechanical defects;

possibility of controlling through chemical composition the refractiveindex, that must be at least a few percent units higher than that ofsurrounding materials;

flat geometry, for easy fit into automated production lines;

thickness of a few μm, preferably about 2 and 20 μm.

In order to ease integration of these devices into production andcommunication lines, the substrate should preferably be made of siliconor silicon oxide.

BACKGROUND OF THE INVENTION

Such devices are presently produced according to physical techniques,among which thermal oxidation of silicon, and those known as Sputtering,Chemical Vapor Deposition and Flame Hydrolysis can be cited. Anothermethod consists in the vacuum deposition on a silicon substrate ofmicroparticles of silicon oxide obtained according to the FlameHydrolysis technique.

However, these productions are complex, requiring costly workingchambers and tools; some of these, such as silicon thermal oxidation,have a limit in the film thickness that can be obtained, while othersare exceedingly slow and are often characterised by low productivity andtoo high costs, so as not to allow an actual industrial exploitation ofoptical devices.

The most economically promising technology for massive production ofglassy films on substrates is sol-gel. Under the name sol-gel aregathered different procedures for the preparation of oxides of one ormore elements in form of porous bodies, ceramics or glasses.

While differing from each other in the specific details, all sol-gelprocedures share the following phases:

preparation of a “sol”, a solution or suspension in water, alcohol orhydroalcoholic mixtures of precursors of the elements whose oxides is tobe prepared. Generally used as precursors are the alkoxides, of formulaM(OR)_(n), where M represents the element whose oxide is desired, thegroup —OR is the alkoxide moiety, and n represents the valence ofelement M; soluble salts of the element M, such as chlorides; nitratesand exceptionally oxides, may be used in place of alkoxides. During thisphase the precursors begin to hydrolyse, that is, alkoxide moieties orother anions bonded to element M are replaced by —OH groups;

sol gelation, requiring from a few seconds up to some days, depending onchemical composition and temperature of the solution; during this phasehydrolysis of the possibly remaining precursor is completed andcondensation occurs, consisting in the reaction of —OH groups belongingto different molecules with formation of one free water molecule and anoxygen bridge between atoms M, M′ (alike or different), according to thereaction:

(HO)_(n−1)M—OH+HO—M′(OH)_(m−1)→(HO)_(n)M—O—M′(OH)_(m)+H₂O   (I)

The product obtained in this phase is called alcogel, hydrogel dependingon the cases, or more generally “gel” as widely used in the Englishliterature.

gel drying; in this phase the solvent is removed by simple evaporationor through hypercritical transformation into gas inside an autoclave;there is obtained an extremely porous dry body, that may have anapparent density ranging from about 10% to about 50% of the theoreticaldensity of the oxide of that composition;

dry gel densification by thermal treating at a temperature generallycomprised between 800° C. and 1200° C. depending on the gel chemicalcomposition and on the parameters of the previous process phases; inthis phase the porous gel densifies obtaining a glassy or ceramiccompact oxide of theoretical density, with a linear shrinkage of about50%.

If gelation phase is not too fast, it is possible to lay a liquid filmof sol on a substrate, eventually resulting in a oxide supported film.Obtaining a oxide film on a substrate in this way is however easilyfeasible only for a thickness up to some tenths of micrometer. Up tosuch values of thickness, cohesive forces in the film are weak, andforces adhering the film on the substrate prevail, so that during thedensification phase there is not in-plane shrinkage of the film anddensification only involves its thickness decrease. At values ofthickness above one micrometer, on the other hand, inner cohesive forcesof the film become prevailing and during densification in-planeshrinking of the film takes place as well: the result is filmfragmentation into “islands” spread over the substrate surface and pooradhesion of the film to the substrate.

This thickness of about 1 μm represents a technological limit forsol-gel technique, as indicated for instance in “SOL-GEL science: thephysics and chemistry of SOL-GEL processing”, Brinker and Scherer,Academic Press, 1990, a comprehensive review of the knowledge in thefield. As already stated above, films prepared in this way are definedthin or thick when they have a thickness below or above about 1 μm,respectively.

For the production of thick films through the sol-gel technique it hasbeen proposed to prepare a sol containing, in addition to normalprecursors, a dense material in the form of nanospheres, that is,spheres of dimensions of about 10 nm. This approach is exposed in thepaper “SOL-GEL derived thick coatings and their thermomechanical andoptical properties”, Menning et al., SPIE Vol. 1758, SOL-GEL Optics II(1992), pages 125-134. This technique however can hardly be implementedpractically; besides, despite the fact that the first papers on thetechnique were published more then five years ago, actual feasibility ofthick films by this route has not been proven yet.

Another proposed approach is to prepare thick films through repeateddepositions of thin films; any single layer must be densified beforedeposition of the subsequent layer. An example of this kind of procedureis given in “Deposition of thick silica-titania SOL-GEL films on Sisubstrates” Syms et al., Journal of Non-Crystalline Solids, 170 (1994),pages 223-233. According to the literature, by this way it is possibleto prepare multilayer thick films. On the other hand, as stated in thecited paper, in order to obtain films of good mechanical and opticalcharacteristics any single layer must have a thickness not higher thanabout 0.25 μm, so that production of a films of thickness about 10 μmrequires about 40 deposition and densification steps.

Thus, the production of large amounts of flat waveguides by the sol-gelroute is still an open problem.

It is thus an object of the present invention to provide a sol-gelprocess for the preparation of thick glassy films of silicon oxide orbased on silicon oxide, as well as to provide glassy supported films ofthickness higher than 1 μm, preferably between 2 and 20 μm.

DISCLOSURE OF THE INVENTION

According to the present invention, these objects are obtained with asol-gel process for the preparation of thick glassy (vitreous) film ofsilicon oxide or mixed oxides containing silicon oxide, comprisingpreparing a sol from a solution or a suspension of precursor elements inwater, alcohol or hydroalcoholic mixtures, said precursor elementscomprising silicon and, optionally, one or more elements selected fromthe group consisting of germanium, aluminum, titanium, and zirconium.The molar ratio of the precursor elements of silicon and the sum of theoptional precursor elements is greater than or equal to 1:1, said solcomprising a water solution and an acid containing at least 10 moles ofH₂O per each mole of said precursor elements and having a pH rangingbetween 0.3 and 1.5 to form a sol. Hydrolysis of the precursor elementsis undertaken, after which about 0.7 to about 3.0 moles of SiO₂ per moleof the precursor elements is added to the sol. A film of the sol isformed on a substrate, and the sol film is gelled via solventevaporation. The resulting gel film is subjected to densificationthrough thermal treatment to form a vitreous film.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail with reference to theaccompanying drawings, in which:

FIG. 1 shows the result of a profilometric test on a sample of theinvention before the densification operation, showing sample thicknessvariations along a line that crosses the film edge;

FIG. 2 shows the result of a profilometric test on the same sample ofFIG. 1 after densification;

FIG. 3 is a schematic view of a sample of the invention obtainedaccording to the interferometric technique in order to put in evidencepossible defects of substrate-film interface;

FIG. 4 shows another image of the same sample and with the same view ofFIG. 3, with the only difference that the image in FIG. 4 has beenobtained according to the “dark field” technique, as explained below.

MODES FOR CARRYING OUT THE INVENTION

In the first phase of the process according to the invention it isprepared on extremely diluted sol of a silicon alkoxide or of a mixtureof alkoxides corresponding to the desired glassy composition. In mixedoxides, the molar ratio between silicon oxide and oxides of otherelements may be 1:1 or higher in the case of germanium, while it isgenerally not lower than 5:1 when elements such as titanium, aluminum orboron are employed.

This sol is preferably of very low concentration and is obtained usingat least 10 mols of H₂O per mole alkoxides, preferably from about 20 toabout 100 mols of H₂O per mole of alkoxides and more preferably fromabout 30 to about 40 mols of H₂O per mole of alkoxides. Preferredalkoxides are those where the alcoholic moiety comes from methyl orethyl alcohol, as these alkoxides are easily hydrolysed and theresulting alcohols may be easily removed by evaporation. Taking siliconas an example, silicon alkoxides may also be defined as silicic acidortho-esthers, and are known in the field as TMOS, that's the acronymfor Tetra-methyl-ortho-silicate, Si(OCH₃)₄, and TEOS, the acronym forTetra-ethyl-ortho-silicate, Si(OCH₂CH₃)₄. H₂O is added as a solution ofan acid of concentration such as to yield a pH value comprised between0.3 and 1.5. The preferred acid is HCl: in this case the acidconcentration is comprised between 0.03 and 0.5 N and preferably between0.04 and 0.2 N.

Alkoxides hydrolysis is an equilibrium reaction; as the inventionprocess necessarily requires that hydrolysis at this stage be completed,and that no traces of alcohol remain in the subsequent phases,hydrolysis is pushed to its stoichiometric completion by distilling theforming alcohol. Distillation is generally performed under pumping,keeping the pressure in the hydrolysis container at a level below 10mbar, and preferably comprised between 3 and 5 mbar. This phase may beaccelerated and favoured by operating at a temperature comprised between30 and 40° C. Hydrolysis is stopped when the volume of alcohol recoveredin a suitable flask is about 110% of the volume of alcoholstoichiometrically produced by alkoxides hydrolysis; recovering anover-stoichiometric volume one takes into account the amount of waterthat may evaporate along with alcohol as an azeotropic mixture, thusensuring total alcohol removal.

To the thus obtained sol there are added from about 0.7 to 3 mols ofSiO₂, preferably about 2 mols of SiO₂ per each starting silicon alkoxidemole. In the preferred embodiment of the invention, SiO₂ compound is inthe form of extremely fine powders, such as the compound obtained byflame hydrolysis. SiO₂ by flame hydrolysis is a particular form ofextremely pure powdery silica, with particles of granulometry of about10 nm, and it is produced feeding SiCl₄ to an oxyhydrogen flame. Thisproduct is commonly available on the market and may be obtained forinstance by Degussa under the tradename Aerosil OX-50. Homogeneousdispersion of flame hydrolysis SiO₂ into the sol may be flavoured bymechanical or ultrasonic stirring.

The thus obtained sol is deposited on a substrate according to knowntechniques, e.g. by means of dip-coating or spin-coating, the first oneconsisting in dipping in and extracting from the sol, at a constantpre-set speed, the substrate kept in vertical position, and the secondone in pouring a pre-set amount of sol on the substrate while spinningthis latter, at a speed generally comprised between 500 and 5000 rpm.

The sol films thus obtained on the substrate are preferably caused togel suddenly through quick solvent evaporation. Gelation consists in thecondensation of —OH groups bonded to different atoms of silicon or ofother possibly present elements, according to reaction (I) given above.Oxygen bridges between two atoms of metal, silicon or germanium areformed, eventually resulting in the formation of an oxidic gel.

Instantaneous gelation is obtained in the simplest way by sudden heatingof the film from room temperature at a temperature of about 300-400° C.,for instance introducing the substrate with the film into a pre-heatedoven. The film may then be left in the oven for a few minutes, toenhance its mechanical strength. Once extracted from the oven, the filmis stable and can be left exposed to air indefinitely. This film isconstituted by a dry gel, having the same chemical composition of thefinal oxide, but with a porous structure.

The last process phase consists in densification of the film, that isrealised in subsequent thermal treatment steps.

As known in the field, the dry porous gel obtained is generallysubjected, as a first preparation step of the densification procedure,to a thermal treatment in oxidising atmosphere, for instance between300° C. and 1000° C., preferably between 500° C. and 800° C. in air oroxygen, in order to remove through combustion the remaining traces oforganic compounds, alcohol or alcoholic moieties, that can have beenleft in the gel pores.

A subsequent step consists in a film dehydration or purificationtreatment, in order to remove the —OH groups possibly remaining in thefilm after gelation, solvent evaporation and combustion removal oforganic moieties. In a first embodiment of the process of the inventionthis is obtained by flowing in the gel pores a gaseous dry dehydratingagent, such as HCl possibly diluted in an inert gas. Alternatively, thesame procedure is realised by using HCl diluted in H₂ in the inert gas.

Once the pre-set temperature in the above said range is reached,substrate and film are kept at such a temperature for a given time,generally comprised between 10 minutes and 1 hour in the presence of adehydrating atmosphere.

Before realising the final densification phase, substrate and film areheated at a temperature comprised between 400° C. and 1000° C.,preferably between 500° C. and 800° C., in a flowing inert gas, such as99.99% pure helium, to wash the film.

The densification phase then involves heating substrate and sample in aflowing inert gas. Specifically, substrate and film are brought totemperatures comprised between 1200° C. and 1400° C. in a 99.99% purehelium during a time preferably comprised between about 10 and about 30minutes.

This process is fully compatible with silicon oxide substrates. When thesubstrate is made of silicon, using HCl mixtures in helium may give riseto microerosions, known in the field as “pittings”, on the samesubstrate surface. To avoid this, it is possible to resort to mixtureswhere the inert gas contains hydrogen along with HCl, with aacid/hydrogen ratio that varies depending on the treatment temperature,according to the conditions indicated in a paper of G. A. Lang,published on RCA Review of 1963, Vol. 24, page 448. This paper showsthat the volume percent of HCl that may be present admixed with hydrogenwithout giving raise to pitting becomes higher the higher thetemperature: as an example, pitting may be avoided with mixturescontaining a HCl volume up to about 1.5% of the volume of hydrogenworking at about 1200° C.; up to about 3% at about 1240° C.; and up toabout 5% at about 1270° C.

Objectives and advantages of the present invention will be betterappreciated by the experts in the field by reading the followingexamples, that are meant to illustrate the invention but by no means areto be considered as limiting its scope. In the Examples from 1 to 5, thepreparation and check of a silicon oxide film on a substrate accordingto the invention is shown, while in Example 6 it is shown thepreparation of a film by using a starting sol of different composition.

EXAMPLE 1 Preparation of a Porous Film on a Substrate

50 grams of TEOS are added to 150 cc of HCl solution 0.1 N in a flask.The thus obtained solution is made homogeneous subjecting it tosimultaneous mechanical and ultrasonic stirring during about 10 minutes.A clear monophasic solution is obtained. The solution is heated at 40°C.; after 1-2 minutes, extraction of ethyl alcohol formed by TEOShydrolysis is begun, maintaining the sol at a temperature of 20° C. inthe flask connected, through a Rotavapor, to a pump that brings thepressure in the reaction flask to about 5 mbar. The condensing pipe ofthe Rotavapor is kept at a temperature of about −20° C. to ensurecomplete condensation of the formed alcohol. The pump is disconnectedfrom the system when in the collecting flask there are measured about 56cc of liquid, essentially consisting of ethyl alcohol. 28.8 grams ofAerosil OX-50 Degussa are added to the thus obtained sol, and themixture is made homogeneous by ultrasonic stirring during 10 minutes. Byusing the thus obtained sol, some films are prepared through thedip-coating technique, dipping and extracting from the sol a siliconsubstrate at a speed of 0.5 cm per second. The sol film isinstantaneously gelled placing it into an oven preheated at 400° C. andkeeping it in the oven for about 10 minutes. On this film, not yetdensified, a profilometric test is carried out by using a Rodenstock RM600 profilometer. This technique allows performance of non-destructivetests to investigate a surface profile; tests may either be performedalong one single line, obtaining the surface heights variations alongthe chosen line, or scanning the surface along parallel lines, thusobtaining the surface heights variations of the whole surface. In thepresent example a single-line mode profilometric test has beenperformed. The result is shown in FIG. 1, reporting film thickness inmicron on the vertical axis and displacement in millimeters on the filmplane on the horizontal axis. The horizontal axis zero value correspondsto the border of the zone reached by the sol during dipping of thesubstrate in the same sol. The resulting film thickness, apart from theedge zone, is of about 10 μm.

EXAMPLE 2 Porous Film Densification

The sample prepared as given in Example 1 is cleaned from traces ofpossibly remaining organic compounds and densified according to thefollowing thermal treatment:

heating from room temperature to 800° C. in helium at an heating rate of4° C. per minute;

treatment in a 10% anhydrous HCl—90% helium mixture during half an hourat 800° C.;

heating in helium up to 1370° C. at an heating rate of 4° C. per minute;

rapid cooling, taking about 6 hours, down to room temperature.

A profilometric tests similar to the one previously described is carriedout on the thus densified film. The test result is represented in FIG.2, similar to FIG. 1, and shows a film thickness of about 8 μm.

EXAMPLE 3 Substrate and Film Check

The dense film sample obtained in Example 2 is inspected with aninterferometric microscope (Zeiss, Mod. AXIOVERT). The results are shownin FIG. 3: focussing the microscope at the interface between theperfectly transparent film and the silicon substrate, black spotscorresponding to silicon surface defects are noted. The image in FIG. 3shows a line, L, representing the edge of film F on substrate S: thesilicon oxide film lays on the upper part of the image.

EXAMPLE 4 Film Check

The same sample of Example 3 is now inspected in “dark field”, using thesame view direction and the same Zeiss AXIOVERT microscope. The “darkfield” technique consists in lighting the sample with light directedtowards the centre of the viewing field and with an incidence degree onthe sample of about 45°. In these conditions, if sample surface has nodefects, light is not reflected in the observation direction and thesample looks black; vice versa, if the sample has defects, these diffuselight in any directions, comprising the observation direction, so thatthe appearance of shining spots or areas in the microscope field revealsa non-perfectly planar surface. The results of this inspection are shownin FIG. 4. It can thus be noted that defects are present on thesubstrate S alone, while film F, corresponding to a zone with no brightspots or zones, results completely free of defects.

EXAMPLE 5 Thick Films Production with No Substrate Defects Generation

A sample obtained according to the procedure of Example 1 is densifiedaccording to the following thermal treatment;

heating from room temperature to 800° C. in oxygen at an heating rate of4° C. a minute;

treating at 800° C. with a gaseous mixture containing one mole of HClper 100 mols of H₂ per 2500 mols of inert gas, such as N₂ or He;

heating in helium up to 1370° C. at an heating rate of 4° C. a minute.

By inspecting the thus obtained sample with the microscope, according toboth the “clear field” and “dark field” techniques, no defects aredetected.

COMPARATIVE EXAMPLE 6

The procedures of Examples 1 and 2 are repeated, with the onlydifference that the HCl concentration for preparing the starting sol islowered at 0.01 N. The result is a broken film showing poor adhesiononto the substrate.

The analysis of tests results shows that the process of the inventionallows the obtainment of thick supported films. The particular, FIG. 2shows that by the invention process a film about 8 μm thick has beenobtained having side dimensions of several millimeters. FIGS. 3 and 4show that, although the substrate surface presents a few point defects(block spots in FIG. 3) the oxidic film formed according to theinvention process has an upper surface with no defects (lack of brightspots in the upper part of the image in FIG. 4, corresponding to thezone where the film is). In this image, defects at the film-structureinterface, that is, under the film, are no longer visible, because inthe “dark field” technique this interface is no longer lighted up beingshielded by the mirror plane represented by the intact film. Siliconsurface defects are avoided if, in the last part of the densificationprocess, a HCl-hydrogen mixture in inert gas instead of HCl alone ininert gas is used, as explained in the cited paper of G. A. Lang and asshown in Example 5. Films obtained according to the process of theinvention are hence endowed with good optical surfaces, allowing theiruse in optics.

Finally, despite the fact that the sol-gel technique has been known andinvestigated for a number of years, and despite the fact that the singlesteps of the present process of the invention may have previouslydescribed in the specialised literature, the process of the inventionallows the obtainment of the above exposed results, that could not beobtained before by the experts in the sol-gel field.

What is claimed is:
 1. A sol-gel process for the preparation of vitreousfilms of silicon oxide or mixed oxides containing silicon oxide,comprising: preparing a sol from a solution or a suspension of precursorelements in water, alcohol or hydroalcoholic mixtures, said precursorelements comprising silicon and, optionally, one or more elementsselected from the group consisting of germanium, aluminum, titanium, andzirconium, wherein the molar ratio of the precursor elements of siliconand the sum of the optional precursor elements is greater than or equalto 1:1, said sol comprising a water solution and an acid containing atleast 10 moles of H₂O per each mole of said precursor elements andhaving a pH ranging between 0.3 and 1.5 to form a sol; hydrolysing saidprecursor elements; adding to said sol about 0.7 to about 3.0 moles ofSiO₂ per mole of said precursor elements; forming a film of said sol ona substrate; gelling the sol film through solvent evaporation, whereinsaid gelling is initiated by introducing said sol film into an ovenpreheated at a temperature from 300° C. to 400° C.; and densifying theresulting gel film through thermal treatment to form a vitreous film. 2.The process according to claim 1, wherein said sol is prepared usingfrom 30 to 40 moles of H₂O per each mole of said precursor elements. 3.The process according to claim 1, wherein a mixture of precursorelements of silicon and germanium are used such that the molar ratio ofsilicon and germanium is equal to or higher than 1:1.
 4. The processaccording to claim 1, wherein a mixture of precursor elements of siliconand of one or more precursor elements selected from the group consistingof titanium, aluminum, zirconium and boron, such that the molar ratio ofsilicon and the sum of said one or more selected precursor elements isgreater than 5:1.
 5. The process according to claim 1, wherein saidprecursor elements are alkoxides of said elements.
 6. The processaccording to claim 1, wherein during sol preparation,tetramethylorthosilicate, Si(OCH₃)₄, or tetraethylorthosilicate,Si(OCH₂CH₃)₄, are used as silicon alkoxides.
 7. The process according toclaim 1, wherein H₂O is added as a solution of HCl of concentrationranging between 0.03 and 0.5N.
 8. The process according to claim 7,wherein H₂O is added as a solution of HCl concentration ranging between0.04 and 0.2N.
 9. The process according to claim 1, wherein hydrolysisof said precursor elements occurs in a hydrolysis container and isdriven towards stoichiometric completion by distillation of alcoholformed in the hydrolysis, keeping the pressure in said hydrolysiscontainer at a value below 10 mbar.
 10. The process according to claim9, wherein pressure in said container is between 3 and 5 mbar.
 11. Theprocess according to claim 9, wherein the temperature in said hydrolysiscontainer is kept at a value between 30° C. and 40° C.
 12. The processaccording to claim 9, wherein hydrolysis is continued until thedistilled volume is 110% of the theoretic volume of alcohol formed inthe hydrolysis.
 13. A sol-gel process for the preparation of vitreousfilms of silicon oxide or mixed oxides containing silicon oxide,comprising: preparing a sol from a solution or a suspension of precursorelements in water, alcohol or hydroalcoholic mixtures, said precursorelements comprising silicon and, optionally, one or more elementsselected from the group consisting of germanium, aluminum, titanium, andzirconium, wherein the molar ratio of the precursor elements of siliconand the sum of the optional precursor elements is greater than or equalto 1:1, said sol comprising a water solution and an acid containing atleast 10 moles of H₂O per each mole of said precursor elements andhaving a pH ranging between 0.3 and 1.5 to form a sol; hydrolysing saidprecursor elements; adding to said sol about 0.7 to about 3.0 moles ofSiO₂ per mole of said precursor elements; forming a film of said sol ona substrate; gelling the sol film through solvent evaporation; anddensifying the resulting gel film through thermal treatment to form avitreous film, wherein said film densifying further comprises: treatingthe film with heat between 500° C. and 800° C. in an oxidizingatmosphere in order to remove through combustion possible traces oforganic compounds, alcohol or alcoholic moieties present in the gel;dehydrating or purifying the film through thermal treatment at atemperature between 500° C. and 800° C. and maintaining the film andsubstrate at said temperature for between 10 minutes and 1 hour in aflow of a gaseous mixture comprising up to 10% by volume HCl in an inertgas; heating the film on the substrate at a temperature between 500° C.to 800° C. in a pure inert gas flow to wash the film; and heating thefilm and the substrate at a temperature between 1200° C. and 1400° C. inan inert gas flow.
 14. The process according to claim 13, wherein amixture of HCl diluted in H₂ in said inert gas is used in dehydrating orpurifying the film.
 15. A process according to claim 13 wherein, whenthe substrate is made of silicon, said gaseous mixture of HCl in aninert gas further contains hydrogen with a molar ratio of 1:100 betweenHCl and hydrogen.
 16. A process according to claim 13, wherein the inertgas is 99.99% pure helium.